From Mauricio Freitas' blog on Geekzone:

During the Great New Zealand Snowstorm 2011 the MetService web site was probably one of the most visited in the country. Unfortunately someone thought it would also be a great time to break into Metservice's online advertising platform and plant a little Trojan horse.

What happened is that when serving the ads the MetService web site was also unknowingly distributing a third party piece of code. This little piece of code would be executed by the users' web browser and silently download and install malware on the visitors' computers.

To make it clear: you wouldn't need to click an ad to load this malware, as its code was being loaded directly from the ad server and executed by your browser. The infection wasn't delivered by an ad. In the delivery method could be applied to any other database driven website. It's not an "online advertising problem".

Full story...